The present invention relates to a pressure control valve, and, more particularly, to an electromagnetic pressure control valve capable of controlling fluid pressure in response to an electric signal.
Pressure control valves are represented by a diaphragm type pressure control valve wherein the surface of the diaphragm of receives feedback pressure and a spool type pressure control valve wherein the end surface of the spool receives feedback pressure. The present invention relates to an electromagnetic pressure-control valve of the spool type.
The spool type electromagnetic pressure-control valve is usually designed in such a manner that a load caused by the electromagnet, a spring load, and an output pressure feedback load are arranged to act on a spool valve which slides in a valve sleeve having a supply port, an output pot, and an exhaust port. Its structure is arranged such that the supply pressure to be outputted is controlled to a level which corresponds to the electric signal supplied to the electromagnet portion by arranging the balance among the above-described loads (see Japanese Patent Laid-Open No. 60-52509).
In the conventional pressure control valves of the type described above, when a pressure governing valve portion comprising a valve sleeve and a spool valve capable of sliding in the valve sleeve and an electromagnet portion comprising a coil assembly, a core, and a plunger are assembled, a flange portion of the valve sleeve and a case covering the electromagnet portion are brought into contact with each other before staking an end portion of the case in such a manner that the contact portion is covered.
However, it is necessary for the flange portion of the valve sleeve and the end portion of the case to be flattened in order to stake the end portion of the case. If a satisfactory flatness cannot be obtained in the two end surfaces, the spool valve accommodated in the valve sleeve and a push rod accommodated in the case via a core lose their concentricity. This leads to a problem in that the operation of the electromagnet portion cannot accurately be transmitted to the spool valve. As a result, the response of the pressure control valve deteriorates or sticking or slippage is caused during the operation of the pressure control valve.
An alternative might therefore be considered of employing a method in order to keep the concentricity between the spool valve and the push rod, the method being arranged so that the plunger is disposed on the other side to the valve sleeve and a flange portion is formed on the core so that the flange portion thus formed and the flange portion formed on the valve sleeve are brought into contact with each other. However, in this case also, the two flange portions may not have sufficiently flat mating surfaces.
Furthermore, it is necessary for the entire surface of the end portion of the flange portions on the valve sleeve and the core respectively taken from casting molds to be subjected to cutting work. As a result, manufacturing cannot be conducted efficiently, and the cost rises excessively.
In the case where the pressure control valve is included in a hydraulic pressure control circuit for use in an automatic transmission, hydraulic oil for operating an oil tank is supplied to the pressure governing valve portion so as to operate the pressure control valve. The electromagnet portion of the pressure control valve is filled with hydraulic oil for the purpose of preventing hydraulic pressure vibrations caused at high temperature. As a result, hydraulic oil for operating the pressure governing valve portion is arranged to be supplied to the electromagnet portion of the pressure control valve.
When hydraulic oil in the oil tank is supplied to the electromagnet portion of the above-described conventional pressure control valve, foreign matter such as iron dust formed in the other bulbs or the like can enter the electromagnet portion together with hydraulic oil.
Such foreign matter adheres to the surface of a plunger since the plunger is magnetized as a result of the supply of an electric signal to the electromagnet portion. The foreign matter accumulated in accordance with the passage of time causes sticking of the plunger or a short circuit of the magnetic circuit. If such foreign matter enters between a push rod for securing the plunger and a liner type rolling bearing, the liner type rolling bearing can be seized by the foreign matter thus caught.
In order to prevent the invasion of such foreign matter into the electromagnet portion, an O ring or the like is disposed between the coil assembly and the case.
However, in the above-described structure arranged such that an O ring or the like is disposed between the coil assembly and the case, a space in which the O ring can be accommodated becomes necessary. Therefore, a recessed portion is formed at the required portion, and finishing is required for the purpose of improving the sealing effect.